Change test release trigger to tags

[RobbeSneyders]

The test release pipeline failed after merging #1454 because you can only upload a package on PyPi with the same version once.

This PR changes the trigger for the test release pipeline to tags, so it only runs when there's a new version.

This way, releasing works as follows:

1. Push tag
2. Test release pipeline
3. Publish release
4. Release pipeline

[coveralls]

Pull Request Test Coverage Report for Build 1809491406

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 97.053%

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Totals
Change from base Build 1809057982:	0.0%
Covered Lines:	2832
Relevant Lines:	2918

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💛 - Coveralls

[Ruwann]

LGTM, another option could be to replace version by the commit SHA when publishing to Test PyPI and still run on every push to main?

[RobbeSneyders]

A commit SHA is unfortunately not a valid version. See PEP 440.